Process for making low cost infrared windows

ABSTRACT

Low cost broadband infrared windows are fabricated using a near net shape process which greatly reduces the cost of machining and grinding window materials. The fabrication of zinc sulfide (ZnS) IR windows uses ceramic powder processing to avoid the expensive prior art chemical vapor deposition method. Further, the invention involves a means of hardening and strengthening ZnS as part of the powder process, with IR transmission performance comparable to undoped CVD-prepared ZnS. The compositional modification used in the practice of the invention involves the introduction of gallium sulfide (Ga 2  S 3 ) as a second phase which acts to toughen and harden the ZnS. The process of the present invention achieves a hardening effect without degrading the IR transmission properties also by means of controlling the polycrystalline microstructure grains to a very small size. At the same time, porosity, which strongly degrades IR transmission, is minimized by full densification. The gallium is introduced into the ZnS by a coprecipitation process to both lower the raw material cost and obtain a suitable submicrometer precursor for the subsequent ceramic processing. The Ga-doped ZnS is then densified into an IR window and a second phase, zinc thiogallate (ZnGa 2  S 4 ), is precipitated out as a hardening phase by heat treatment. Alternatively, Ga metal is evaporated onto densified ZnS and subjected to heat treatment to form the zinc thiogallate phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to broadband infrared windows,and, more particularly, to a low cost process for manufacturing suchwindows.

2. Description of Related Art

Recent efforts have been undertaken to develop a vision enhancementsystem for automotive and other high volume applications. Such systemsemploy infrared (IR) detectors and are used, for example, to provide anearly warning of close proximity to other vehicles.

Infrared emitter and detector assemblies commonly employ a window whichprotects the infrared components while permitting the transmission ofinfrared radiation. Commonly, II-VI materials, such as zinc sulfide(ZnS), zinc selenide (ZnSe), and cadmium telluride (CdTe), have beenemployed as infrared window materials.

ZnS windows were produced by hot pressing of ceramic powders years agoby Kodak and others; see, e.g., U.S. Pat. No. 3,131,238, issued Apr. 28,1964. The development of a chemical vapor deposition (CVD) process forfabricating ZnS largely replaced the prior method of making ZnS formilitary applications where cost was not an important consideration. TheCVD Zns could be fabricated in larger, flat or curved pieces to conformto aircraft shape requirements. However, there was no improvement inmechanical properties with the CVD process. In fact, when CVD ZnS wasannealed, it became more transmissive in the optically visible region,but the ZnS also became much softer. It is believed that the removal ofthe high temperature (hexagonal) wurtzite phase by the heat treatment isresponsible for the increase in IR transmission and that the growth oflarge grains caused the material to soften. For a discussion of ZnSformation by hot pressing and by CVD, see J. A. Savage, Infrared OpticalMaterials and Their Antireflection Coatings, Adam Hilger LTD., Bristoland Boston (1985), pp. 95-111.

The CVD process is very slow and not easily amenable to high volumeproduction, and typically requires a week to make a CVD run. Theas-formed CVD ZnS has a yellow color due to the presence of absorbingdefects. But most important is the intrinsic high cost of CVDfabrication which makes IR windows produced by this method prohibitivelyexpensive for high volume markets.

The use of gallium sulfide for hardening zinc sulfide was investigatedby J. Zhang et al, "Solid-State Phase Equilibria in the ZnS-Ga₂ S₃System", Journal of the American Ceramic Society, Vol. 73, No. [6], pp.1544-1547 (1990). It was found that ZnS-Ga₂ S₃ solid solution yieldedmore than a 50% increase in hardness and fracture toughness. A model wasdeveloped that correlated porosity and second phase material (ZnGa₂ S₄,zinc thiogallate) with transmission in the infrared region. The modelpredicted that when porosity approached zero volume percent andprecipate sizes were smaller than one micrometer, the transmissionproperties of zinc sulfide would not be altered.

The work of Zhang et al was limited to the addition of gallium sulfidein the bulk zinc sulfide material. They published additional work on thephase equilibria of the gallium and zinc sulfide binary system toestablish a means for forming zinc thiogallate as a second phase. Thesolid solution region for gallium in zinc sulfide was found to decreasewith lower temperatures below the cubic to hexagonal phasetransformation temperature at 1,025° C. Thus, at a lower temperature,the gallium would be expected to precipitate out as a second phase, zincthiogallate (ZnGa₂ S₄) as described by W. W. Chen et al, "Experimentaland Theoretical Studies of Second-Phase Scattering in IR TransmittingZnS-Based Windows", Proceedings of SPIE, San Diego (1991).

A low cost IR window is a mandatory requirement in order to be able tomarket an affordable vision enhancement system for automotive and otherhigh volume applications. The cost of the IR window is a major factor inthe viability of offering an IR device for general use. The potentialsafety benefits to the driving public are enormous. Many lives could besaved and needless destruction of automobiles could be prevented if sucha device were available at a reasonable cost.

Accordingly, a need exists for the production of a low cost IR window.

SUMMARY OF THE INVENTION

In accordance with the invention, a method is provided for thefabrication low cost broadband infrared windows. The window fabricationmethod involves a near net shape process which greatly reduces the costof machining and grinding window materials. The fabrication of zincsulfide (ZnS) IR windows uses ceramic powder processing to avoid theexpensive CVD method. Further, the invention involves a means ofhardening and strengthening ZnS as part of the powder process, with IRtransmission performance comparable to undoped CVD-prepared ZnS.

The compositional modification used in the practice of the inventioninvolves the introduction of gallium sulfide (Ga₂ S₃) as a second phasewhich acts to toughen and harden the ZnS. The invention achieves ahardening effect without degrading the IR transmission properties alsoby means of controlling the precipitates to a very small size. At thesame time, porosity, which strongly degrades IR transmission, isminimized by full densification. In one embodiment, the gallium isintroduced into the ZnS by a coprecipitation process to both lower theraw material cost and obtain a suitable submicrometer precursor for thesubsequent ceramic processing. The Ga-doped ZnS is then densified intoan IR window and a second phase, zinc thiogallate (ZnGa₂ S₄), isprecipitated out as a hardening phase by annealing. Alternatively,gallium metal is deposited on the densified ZnS prior to annealing. Theannealing process then forms the zinc thiogallate phase.

The process of the present invention comprises:

(a) either

(1) forming a coprecipitate of a sulfide, a zinc salt, and a galliumdopant in a liquid medium,

(2) pressing the coprecipitate in a die, and

(3) densifying the pressed coprecipitate by hot isostatic pressing toform a densified body;

(b) or

(1) forming a coprecipitate of a sulfide and a zinc salt in a liquidmedium,

(2) pressing the coprecipitate in a die,

(3) densifying the pressed coprecipitate by hot isostatic pressing toform a densified body, and

(4) depositing a layer of gallium metal on at least one surface of thedensified body; and

(c) annealing the densified body.

Either in the coprecipitation step or following densification, thegallium dopant is introduced into the material. If introduced in thecoprecipitation step, a gallium salt is combined with the sulfide andzinc salt. If introduced after densification, evaporation of galliummetal onto the surface of the densified ZnS is performed, followed byannealing to diffuse it to a depth dependent on the time and temperatureof annealing.

The principal benefits of the present invention are the attainment of alow cost IR window with improved strength while maintaining the same IRtransmission property. The lower cost is achieved mainly through the useof ceramic powder processing with higher potential production volume,shorter cycle time processing steps, by eliminating the need forexpensive CVD processing, by forming to near net shape to eliminatemachining, and by reducing the need for a hard coating to protect therelatively soft and weak ZnS IR window material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, on coordinates of transmission and wavelength (in micrometers),is a plot of the transmission curves of a hypothetical 2 mm thick ZnSparallel slab containing 0.3 micrometer pores as a function of volumefraction porosity;

FIG. 2, on coordinates of transmission and wavelength (in micrometers),is a plot of the transmission curves of a hypothetical 2 mm thick ZnSparallel slab containing a fixed 5 mole percent as a function of size ofZnGa₂ S₄ particles;

FIG. 3, on coordinates of transmission and wavelength (in micrometers),is a plot of the transmission curves of a hypothetical 2 mm thick ZnSparallel slab containing approximately 500 Å (50 nm) radius ZnGa₂ S₄particles as a function of mole percent;

FIG. 4, on coordinates of transmission and wavelength (in micrometers),is a plot of infrared transmittance of ZnS over a wavelength of 2 to 14μm after hot pressing and then after hot isostatic pressing inaccordance with the invention compared with ZnS formed by a prior artchemical vapor deposition process; and

FIG. 5, on coordinates of transmission and wavelength (in micrometers),is a plot of infrared transmittance of ZnS over a wavelength of 2 to 14μm after hot isostatic pressing and gallium surface toughening inaccordance with the invention compared with ZnS formed by a prior artchemical vapor deposition process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The model developed by Chen and Dunn, supra, for correlating porositywith transmission in the infrared region is depicted in FIG. 1. Thismodel predicts that with porosity approaching zero and with ZnGa₂ S₄precipitates smaller than 1 micrometer, the addition of gallium sulfidewould not degrade the transmission properties as depicted in FIGS. 2 and3.

Specifically, in FIG. 1, the importance of eliminating residual porosityis illustrated in the plot of transmission vs. IR wavelength. Thetransmission curves are all calculated for hypothetical ZnS window withtheoretical densities ranging from 99.00 to 99.99%.

FIG. 2 illustrates the effect of pore size on transmission. A pore sizeof 1 μm causes so much scattering that nearly all IR transmission below8 μm in wavelength is eliminated. However, when the pore size is assmall as 0.025 μm, the transmission is nearly at the theoretical levelof 70 to 75%.

FIG. 3 shows the IR transmission vs. wavelength for 1, 2, and 5 molepercent theoretical additions of zinc thiogallate. While this plot is acalculated curve, it shows that the IR transmission can still bemaintained even with the presence of a zinc thiogallate second phase.

In the process of the present invention, gallium is added for hardeningzinc sulfide by coprecipitation of the gallium as an integral part ofthe zinc sulfide crystal lattice which forms an intimate mixture of zincsulfide and gallium sulfide. The (Zn,Ga)S solid solution can then bemixed with the pure ZnS as a bulk component, a surface enrichmentcomponent, or as part of a gradient of concentrations.

The prior art CVD process for making IR windows and ZnS windows inparticular is expensive, costing $40 to $80 per window (1993 dollars),is slow, and results in low volume production. In contrast, the processof the invention is comparatively cheap, costing $15 to $25 per window,is fast, and results in high volume production. In addition, the capitalcost of equipment to scale-up to high volume production is much lesswith the process of the present invention compared with that for the CVDprocess.

The prior art CVD process begins with molten Zn and gaseous hydrogensulfide (H₂ S). These reactants are used to form a ZnS window in arather slow deposition process, taking about 1 week. The resulting CVDZnS is then annealed, machined to size, and ground and polished. A hardcoating is then applied to the windows, followed by application of ananti-reflective coating. The chemical vapor deposition is the ratelimiting step in the process.

The near net shape (powder) process in the present invention involvesfirst a coprecipitation to form a very small particle sized grain of ZnS(less than 0.2 μm). The reactants used in the preferred embodiment ofthe invention are sodium sulfide and zinc acetate in an aqueoussolution. However, other sulfides, such as ammonium sulfide, lithiumsulfide, potassium sulfide, and hydrogen sulfide, and other zinc salts,such as zinc nitrate, zinc chloride, and zinc sulfate, may be employedin the practice of the present invention.

By controlling the pH, concentration, temperature, product removal rate,and mixing conditions, the particle size distribution can be controlled.The process of the present invention avoids the need for H₂ S injectioninto the reactor vessel. However, H₂ S is produced as a by-product whichis collected and neutralized using a dilute sodium hydroxide solution.The chemical reaction can be written as follows:

NaS+Zn(acetate)₂ →ZnS↓+Na(acetate) then, ZnS+H₂ O→ZnO+H₂ S↑.

The concentration of the sulfide and zinc salt each ranges from about0.02 to 0.5 Molar. A tradeoff ensues: the lower concentrations promotesmaller grain sizes of the coprecipitate, while the higherconcentrations promote higher yields. This is due to mechanical lossescaused by manipulation of large quantities of aqueous solutions. Theoptimum concentration that balances these considerations is about 0.4Molar. For stoichiometric reaction, the sulfide and zinc salt arepreferably employed in the same concentrations, so as to provide a ratioof 1:1 of atomic sulfur to atomic zinc.

The coprecipitation process lends itself to the introduction of adopant, gallium (Ga). Ga has been shown to significantly improve the ZnSmechanical properties of fracture toughness and the hardness by as muchas 50 percent.

Gallium is introduced by adding about 1 to 9 mole percent of a galliumsalt, such as a nitrate, acetate, chloride, or sulfate of gallium, inthe mixture of the sulfide and zinc salt. The gallium additive mixeswith the zinc salt in the appropriate ratio. The resulting precipitateis concentrated with a centrifuge and cleansed of the sodium acetateby-product by alternate water washings and centrifuging operations.

The subsequent steps in the process include pressing in a die to thedesired shape, with an allowance for the expected shrinkage duringsintering if pressed cold. Hot pressing does not result in anysignificant shrinkage so its die can be the net shape and size of thedesired window configuration.

Hot pressing is performed at a temperature ranging from about 500° to980° C. at a pressure ranging from about 2,000 to 25,000 psi (140.6 to1,757.5 Kg/cm²) for a time ranging from about 5 to 240 minutes. The hotpressing is performed in a vacuum, such as pulled by a mechanical pump,which is on the order of 50 milliTorr or below.

At a temperature less than about 500° C., the material is not asplastic, and does not form a fully dense material. At the uppertemperature end, there is a phase transition at about 1,020° C. fromcubic to hexagonal that must be avoided.

A pressure less than about 2,000 psi (140.6 Kg/cm²) results in a compactthat has open pores and is less dense, while a pressure greater thanabout 25,000 psi (1,757.5 Kg/cm²) requires expensive dies and is notnecessary in the practice of the process of the present invention.

In an alternative process to hot pressing, the coprecipitated powdersmay be cold pressed and then vacuum sintered. Cold pressing is performedat a temperature ranging from room temperature to about 300° C. at apressure ranging from about 5,000 to 40,000 psi (351.5 to 1,212 Kg/cm²)for about 2 minutes. It is preferred that moisture be excluded from thepowder after drying and during pressing. Vacuum sintering is performedat a temperature ranging from about 600° to 850° C. for a period of timeranging from about 10 minutes to 2 hours.

An additional feature of the invention is the use of polished dieplatens so that the finish is replicated to greatly minimize grindingand polishing. The die platens are polished using a hard material downto 0.5 μm size. By forming to net shape and size, the initial machiningrequired for CVD processing is eliminated.

The porosity is removed by densifying to full theoretical density by hotisostatic pressing (HIP) at a temperature ranging from about 800° to1,015° C. at a pressure ranging from about 5,000 to 60,000 psi (351.5 to4,218 Kg/cm²) for a period of time ranging from about 5 to 360 minutesin an inert atmosphere, such as argon or nitrogen.

The desired zinc thiogallate phase is then achieved by subsequentannealing. The annealing can be accomplished during the hot isostaticpressing or, in the interest of maximizing the production utilization ofthis equipment, the annealing can be accomplished in a separateatmosphere controlled furnace. The annealing is performed at atemperature ranging from about 550° to 850° C. for a period of time fromabout 1 to 4 hours. Oxidation of the zinc sulfide windows is preventedby the use of an inert or reducing atmosphere; argon is an example of asuitable atmosphere.

The grain size can be kept small by using lower pressing temperaturesand shorter soak times. The tradeoff is the minimum temperature and timerequired to achieve fully dense ZnS with acceptable IR transmission.

In an alternate embodiment, the gallium dopant is not combined in thecoprecipitation step. Rather, only the sulfide and zinc salt arecoprecipitated, and the precipitate hot pressed and then hot isostaticpressed. Following this densification, gallium metal is evaporated onthe surface of the ZnS body, followed by annealing under the conditionsdescribed above to diffuse the gallium metal to a depth dependent on thetime and temperature, thereby forming a surface zinc thiogallate phase.

The overall process of the invention is thus seen to be considerablyshorter than the prior art CVD process.

The resulting improvements in IR transmission from the invention afterthe HIP removes the porosity remaining from hot pressing are shown inFIG. 4. Curves 10 and 12 depict the result of IR windows with noanti-reflection (A-R) coating employing portions of the process of theinvention. Curve 10 compares the transmittance of an IR window followingthe HIP cycle with the transmittance of an IR window after hot pressingonly (Curve 12), both measured on the same sample. The transmittance ofa CVD-processed IR window is included for comparison (Curve 14). Thegoal was 60% transmittance without the A-R coating. The HIP cycleincorporated an annealing step as part of the cycle. The anneal involvedholding the sample at 550° C. for two hours during the cooldown of theHIP furnace.

The effect of surface hardening with Ga-doping in the present inventionis to actually increase the IR transmission. It is not clear whether aslight reduction in the index of refraction is responsible for thisimprovement, shown in FIG. 5. Curve 16 demonstrates the improvement withthe zinc thiogallate second phase, generated by annealing.

Specifically, the lower curve (Curve 18) was derived from commerciallyprepared ZnS powder, available from Aldrich Co., that was hot pressed,hot isostatic pressed, and then polished. The next higher transmittingcurve (Curve 16) was the resulting transmission after the same sampleused to generate Curve be was subjected to evaporation of Ga metal,followed annealing in a fused quartz tube to cause the Ga to diffuseinto the ZnS and form zinc thiogallate. The annealing was done at atemperature of 650° C. in an inert (argon) atmosphere. The fact that theIR transmission was improved by about 10% over the entire spectrum mayhave been due to the formation of a layer having a lower index ofrefraction. However, most importantly, the IR transmission at least wasnot degraded by the hardening process.

The improvements in hardness are shown in Table I for bulk tougheningwith gallium thiogallate.

                  TABLE I                                                         ______________________________________                                        Microindentation Hardness Measurements for ZnS.                               Material      Process     Hardness (GPa)                                      ______________________________________                                        II-VI - undoped                                                                             CVD - baseline                                                                            1.95 ± 0.03                                      Untreated     Hot pressed 1.92 ± 0.03                                      Untreated     HIP'd       2.44 ± 0.02                                      1% bulk doping                                                                              HIP'd       2.54 ± 0.05                                      9% bulk doping                                                                              HIP'd       3.08 ± 0.24                                      ______________________________________                                    

The experimental work has reduced the present invention to practice bydemonstrating the process. Measurable improvements have been observed inIR transmission and hardness as a result of using the preferredembodiment of this invention.

In the present invention, gallium was added either as a surfacediffusing ion or as a surface coating. Scanning electron microscopy(SEM) was used to analyze the microstructure of surface doped ZnS. Theextent of diffusion by gallium into the ZnS as determined by electrondispersive x-ray (EDAX) ion map overlay of the same region was found tobe to a depth approximately 40 micrometers. An alternative lower costmethod for introducing gallium in the present invention is that of thecoprecipitation of gallium as an integral part of the zinc sulfidecrystal lattice which forms an intimate mixture of zinc sulfide andgallium sulfide. The zinc and gallium sulfides can then be mixed withthe pure ZnS as a bulk component or as a surface enrichment component.

EXAMPLES Example 1

0.7 mol (153 g) of zinc acetate (zinc acetate dihydrate, SpectrumZ1045), 0,028 mol (10.2 g) of gallium nitrate (gallium nitratehexahydrate, Johnson Mathey 11150) in 1 liter of degassed H₂ O werecombined with 0,728 (175 g) of sodium sulfide (sodium sulfidenonahydrate, Spectrum S1465) in 1 liter of degassed H₂ O and then addedto 2.5 liters of degassed H₂ O with mechanical stirring. The additionperiod lasted one hour. Care was taken to always have an excess of thebasic solution (sulfide) in the precipitation medium. The powder wasisolated by concentration of the slurry in two 1 liter buckets on thecentrifuge (about 15 minutes at 3,000 rpm). The concentrated powderswere washed 5 times with degassed H₂ O by successive shaking andcentrifuging.

Hot pressing was performed in the range of 800° to 950° C. with dwelltimes of 30 to 180 minutes. The hot isostatic pressing conditionsemployed were 825° to 995° C. with pressures of 25,000 to 30,000 psi(1,757.5 to 2,109 Kg/cm²) and dwell times of 20 to 120 minutes. Due toagglomeration problems, near theoretical density was not achieved.However, the hardness values typically obtained are shown in Table I.

Example 2

1.46 mol (320 g) of zinc acetate (zinc acetate dihydrate, SpectrumZ1045) in 1.5 liter of degassed H₂ O and 1.5 mol (360 g) of sodiumsulfide (sodium sulfide nonahydrate, Spectrum S1465) in 1.5 liter ofdegassed H₂ O were both stirred into 2 liters of H₂ O. The remainingprecipitation and densification steps were performed as in Example 1.The hardness values typically obtained are shown in Table I.

Thus, there has been disclosed a method of making infrared windows. Itwill be readily apparent to those skilled in this art that variousmodifications and changes of an obvious nature may be made withoutdeparting from the scope of the invention, and all such modificationsand changes are considered to fall within the scope of the invention, asdefined by the appended claims.

What is claim is:
 1. A process for making an infrared window comprisingthe steps of:(1) mixing a sulfide, a zinc salt, and a gallium dopant ina liquid medium, and forming a coprecipitate comprising zinc sulfide andsaid gallium dopant (2) pressing the coprecipitate in a die, (3)densifying the pressed coprecipitate by hot isostatic pressing to form adensified body; and (4) annealing the densified body.
 2. The process ofclaim 1 wherein said coprecipitate is formed by mixing said sulfide,said zinc salt, and said gallium dopant in an aqueous medium.
 3. Theprocess of claim 2 wherein said sulfide is selected from the groupconsisting of sodium sulfide, ammonium sulfide, lithium sulfide,potassium sulfide, and hydrogen sulfide and said zinc salt is selectedfrom the group consisting of zinc acetate, zinc nitrate, zinc chloride,and zinc sulfate.
 4. The process of claim 3 wherein said sulfideconsists essentially of sodium sulfide and said zinc salt consistsessentially of zinc acetate.
 5. The process of claim 2 wherein theamount of each of said sulfide and zinc salt ranges from about 0.02 to0.5 Molar, the ratio of sulfide to zinc salt being approximately 1:1 ofatomic sulfur to atomic zinc.
 6. The process of claim 5 wherein saidamount of each of said sulfide and zinc salt is about 0.4 Molar.
 7. Theprocess of claim 2 additionally comprising mixing a gallium dopant withsaid sulfide and said zinc salt.
 8. The process of claim 7 wherein saidgallium dopant is selected from the group consisting of gallium nitrate,gallium acetate, gallium chloride, and gallium sulfate.
 9. The processof claim 8 wherein said gallium dopant consists essentially of galliumnitrate.
 10. The process of claim 7 wherein the amount of said galliumdopant ranges from about 1 to 9 mole percent.
 11. The process of claim10 wherein said amount of said gallium dopant is about 5 mole percent.12. The process of claim 1 wherein said coprecipitate is prepared forpressing by centrifuging to remove substantially all said liquid medium.13. The process of claim 1 wherein said pressing is either performed ata temperature ranging from about 500° to 980° C. at a pressure rangingfrom about 2,000 to 25,000 psi (140.6 to 1,757.5 Kg/cm²) for a period oftime ranging from about 5 to 240 minutes in a vacuum of about 50 mTorror less or performed at a temperature ranging from room temperature toabout 300° C. at a pressure ranging from about 5,000 to 40,000 psi(351.5 to 2,812 Kg/cm²) for a period of time of about 2 minutes,followed by vacuum sintering of said coprecipitate at a temperatureranging from about 600° to 850° C. for a period of time ranging fromabout 10 minutes to 2 hours.
 14. The process of claim 1 wherein said hotisostatic pressing is performed at a temperature ranging from about 800°to 1,015° C. at a pressure ranging from about 5,000 to 60,000 psi (351.5to 4,218 Kg/cm²) for a period of time ranging from about 5 to 360minutes in an inert atmosphere.
 15. The process of claim 1 wherein saidannealing is performed at a temperature ranging from about 550° to 850°C. for a period of time ranging from about 1 to 4 hours in an inert orreducing atmosphere.
 16. A process for making an infrared windowcomprising the steps of:(a) mixing sodium sulfide, zinc acetate, andgallium nitrate dopant in an aqueous medium and forming a coprecipitatecomprising zinc sulfide and said gallium dopant; (b) pressing thecoprecipitate in a die at a temperature ranging from about 500° to 980°C. at a pressure ranging from about 2,000 to 25,000 psi (140.6 to1,757.5 Kg/cm²) for a period of time ranging from about 5 to 240 minutesin a vacuum of about 50 mTorr or less; (c) densifying the pressedcoprecipitate by hot isostatic pressing performed at a temperatureranging from about 800° to 1,015° C. at a pressure ranging from about5,000 to 60,000 psi (351.5 to 4,218 Kg/cm²) for a period of time rangingfrom about 5 to 360 minutes in argon; and (d) annealing the densifiedcoprecipitate at a temperature ranging from about 550° to 850° C. for aperiod of time ranging from about 1 to 4 hours in an inert or reducingatmosphere.
 17. The process of claim 16 wherein the amount of each ofsaid sodium sulfide and zinc acetate is about 0.4 Molar, and wherein theamount of said gallium nitrate dopant is 1 to 9 mole percent.
 18. Theprocess of claim 16 wherein said coprecipitate is prepared for pressingby centrifuging to remove substantially all said liquid medium.